Photon counting may be employed in a number of contexts, one of which is medical imaging. For example, energy resolving photon counting detectors are used for spectral computer tomography (CT). Somewhat similar detectors are provided also in other applications, e.g. photon-counting mammography.
One of the challenges in photon counting is the so-called charge sharing. In particular, charge sharing in direct-conversion materials causes a degradation of the spectral performance. A single X-ray interaction in the sensor may cause charge to drift to two or more pixels depending on the location of the charge cloud within the bulk. The charge is therefore distributed over two or more pixels and is conventionally registered as two or more events with corresponding lower equivalent energy. Effectively this means that the expected spectrum is contaminated by false low energy events which no longer contribute to the photo-peak but rather show up as a background signal generally referred to as “low energy tail”.
Reducing or completely removing such low energy spurious events has a significant impact on improving the spectral quality of the detector and will result, for example, in improved, i.e. less noisy, material decomposed images in the context of spectral (photon-counting) CT.
An approach on charge sharing compensation led to the Medipix3-CERN ASIC (see, for example, “The Medipix3 Prototype, a Pixel Readout Chip Working in Single Photon Counting Mode With Improved Spectrometric Performance” by Ballabriga, R. et al., IEEE Transactions on Nuclear Science (Volume: 54, Issue: 5), October 2007).
US 2012/0112088 A1 is also related to photon detection and provides for correcting charge sharing in a radiation scanner. A data acquisition component of US 2012/0112088 A1 may be configured to determine whether charge sharing has occurred based upon an identified energy spectrum of one or more pulses, or based upon which thresholds are activated and/or deactivated for a given pulse within a given time frame.
According to the conventional approaches a (in some cases even significant) reduction in count-rate performance may occur.
Detection of a charge sharing based on an energy spectrum or based on the threshold history of the pulse involves complex considerations can only be performed after the complete pulse.
A counting digital x-ray detector for recording x-ray images of an object irradiated by x-ray radiation according to US 2014/0175299 A1 includes a direct x-ray converter for converting x-ray radiation into an electric signal and a matrix with a plurality of counting pixel elements. At least one part of the counting pixel elements has a signal input and two circuits for converting the signal into a count signal. A first circuit of the two circuits is configured to convert the signal entering the respective pixel element directly into a count signal and to count the count signal. A second circuit of the two circuits is configured to convert the signal entering directly into the resective pixel elemnet together with coincident occurring signals of at least one neighboring pixel element into a count signal and to count the count signal. The first circuit and/or the second circuit are able to be activated individually and both together.
WO 2004/064168 A1 provides a photon-counting imaging device for single x-ray counting comprising: a) a layer of photosensitive material; b) an NxM array of photodetector diodes arranged in said layer of said photosensitive material; c) an NxM array of readout unit cells comprising an high gain, low noise amplifying means, one readout unit cell for each photodetector diode; the readout unit cells being controlled by a data processing means; d) each readout unit cell comprising an internal data processing means allowing to assign an output signal representing an amplifiyied signal of the electron hole pairs generated by an incident photon or a predetermined number of incident photons in the respective photodectector diode to a preselectable region of interest: and e) said assignment of the output signal is accompanied by a time stamp generated by a clock means. Due to this measures beside the normal operation of the photon counting imaging device the output signal generated by the anplifying means can be treated separately with respect to the occurrence of the photon hitting the respective photodetector diode. For that reason, the time resolution at least for this preselected region of interest is enhanced as far as the clock means allow this resolution with an appropriate frequency.